In recent years, new types of laser using a photonic crystal have been developed. A photonic crystal consists of a dielectric matrix body in which an artificial periodic structure is created. Usually, the periodic structure is created by providing the matrix body with a periodic arrangement of areas whose refractive index differs from that of the material of the matrix body (this area is called the “modified refractive index area”). The periodic structure causes a Bragg diffraction within the crystal and creates an energy band gap in the energy of light that exists in the matrix body. There are two types of photonic crystal lasers; one of which utilizes a band-gap effect to use a point-like defect as a resonator, and the other utilizes a standing wave at a band edge where the group velocity of light becomes zero. Each of these photonic crystal lasers causes an oscillation of laser light by amplifying light of a predetermined wavelength.
Patent Document 1 discloses a two-dimensional photonic crystal surface emitting laser in which a two-dimensional photonic crystal is created in the vicinity of an active layer containing a light-emitting material. This two-dimensional photonic crystal is created from a plate-shaped matrix body made of a semiconductor in which areas such as holes whose refractive index differs from that of the matrix body are periodically arranged. This period distance is adjusted so that it equals the wavelength of the light within the two-dimensional photonic crystal, the light being generated in the active layer. Therefore, the light having that wavelength is intensified to create a laser oscillation. The laser light thus created is diffracted by the two-dimensional photonic crystal perpendicularly to it, which generates a surface emitting light from the surface of the two-dimensional photonic crystal.
The reason why light is amplified in a two-dimensional photonic crystal will be explained using a concrete example of a two-dimensional photonic crystal illustrated in FIG. 1. FIG. 1A is a plain view of a two-dimensional photonic crystal and FIG. 1B is a perspective view of it. This two-dimensional photonic crystal 11 consists of a plate-shaped matrix body 12 in which cylindrical holes 13 are arranged in a square lattice pattern. This square lattice has a period distance “α” which equals the wavelength of the light introduced into the two-dimensional photonic crystal 11 from the active layer. The light introduced into the two-dimensional photonic crystal 11 propagates within the two-dimensional photonic crystal 11 and is reflected 180 degrees by the holes 13 (FIG. 1A). If only one hole 131 is observed, the optical path difference between the light reflected 180 degrees at the hole 131 and the light reflected 180 degrees at the hole 132 which is adjacent to the hole 131 is “2α”, i.e. two times as long as these lights' wavelength. Therefore, the lights are intensified by interference. Repeated interferences at each hole 13 amplify lights in the two-dimensional photonic crystal 11. This phenomenon is called the feedback effect. Some of the lights amplified are diffracted in a direction perpendicular to the surface of the two-dimensional photonic crystal 11 by the holes 13 (FIG. 1B). Since the optical path difference between the lights reflected in a perpendicular direction at the holes 131 and 132 which are adjacent to each other is “α”, i.e. as long as the wavelength of these perpendicularly reflected lights, the lights emitted in a perpendicular direction are also amplified by an interference. Such amplification action generates surface emitting laser light in a direction perpendicular to the surface of the two-dimensional photonic crystal 11.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-332351 (Paragraphs [0037] to [0056], FIG. 1)
It can be thought that, in a two-dimensional photonic crystal surface emitting laser, the larger the area of an active layer and that of a two-dimensional photonic crystal become, the larger the emission area becomes, which yields higher output. However, if a two-dimensional photonic crystal's area becomes larger, the number of the modified refractive index areas which surround a modified refractive index area located in the vicinity of the center region of the two-dimensional photonic crystal becomes larger in comparison to the number of the modified refractive index areas which surround a modified refractive index area located on the edge of the two-dimensional photonic crystal. Hence, the feedback light's intensity increases in the vicinity of the center of the two-dimensional photonic crystal, which makes lights in the two-dimensional photonic crystal more likely to be localized in the vicinity of the center.
If the localization of lights occurs as just described, it is not possible to generate laser light having a single wavelength since electric charges externally injected for causing the active layer to emit light are spatially distributed in the two-dimensional photonic crystal and the refractive index of the matrix body is hence spatially distributed as well. If the feedback effect is reduced in some way, such a localization of lights can be abated; however, if the feedback effect is reduced too much, then a laser oscillation will not be generated. Given this factor, it is necessary to adjust the feedback effect so that these two conflicting effects are balanced.
Conventionally, however, how to reduce or increase the feedback effect in a two-dimensional photonic crystal has not been studied.